1. Field of the Invention
This invention relates generally to current mirror circuits and more particularly to such circuits which are suitable for manufacture in bipolar integrated circuit form. 2. Description of the Prior Art
Some early types of operational amplifiers, for instance, include first and second differentially connected NPN transistors having a PNP current mirror functioning as a differential-to-single ended converter connected between the collectors thereof. The current mirror includes a diode-connected PNP transistor and a further PNP transistor. The diode-connected transistor has an emitter electrode connected to a positive supply conductor, a collector electrode connected to the base electrode thereof and to the collector electrode of the first differential NPN transistor. The further PNP transistor has an emitter electrode connected to the positive supply conductor, a base electrode connected to the base electrode of the diode-connected transistor, and a collector electrode connected to the collector electrode of the second differential NPN transistor. The purpose of the current mirror in this configuration is to convert the differential output signals of the differential pair of NPN transistors into a single ended signal and to enhance the common mode rejection ratio of the differential pair.
The above-described amplifier also includes an output or second stage PNP transistor with an emitter electrode connected to the positive supply conductor, a base electrode connected to the collector electrode of the second differential NPN transistor and a collector electrode connected through a current source or supply to the negative supply conductor. Another current supply is connected between emitter electrodes of the first and second differentially connected NPN transistors and the negative supply conductor. A frequency compensation capacitor is connected from the collector electrode of the output PNP transistor to the collector electrode of the second NPN transistor and the output signal is taken from the collector electrode of the PNP output transistor.
The process and geometry generally utilized in manufacturing the PNP transistors commonly used in such bipolar integrated circuitry results in such devices having poor frequency responses. Thus, the PNP differential-to-single ended converter utilized in the above-described amplifier tends to have a poor frequency response which results in phase and stability problems. Hence, the PNP second stage of the above-described amplifier also has a poor frequency response which is difficult to stabilize by the compensation capacitor connected between the collector and base thereof.
As the requirements for improved frequency response increased, it became apparent that the second stage transistor of such integrated amplifier circuits must be of an NPN configuration to facilitate improved frequency response and stability. Thus, the conductivity types of the transistjors and the supply polarities of the above-described amplifier were reversed. Accordingly, PNP differentially connected input transistors having emitter electrodes connected through a current supply to the positive supply conductor are presently utilized. An NPN current mirror is connected between the collector electrodes of the differential transistors and the negative supply. An NPN output transistor is coupled between the current mirror and the amplifier output terminal. A further current supply is connected between the positive supply conductor and the collector electrode and a frequency compensating capacitor is connected between the collector electrode and base electrode of the output NPN transistor. This particular improved amplifier configuration is used in many present day applications because the frequency compensating capacitor effectively frequency compensates the NPN output transistor. Unfortunately, the frequency response of the amplifier configuration is generally limited to below 1 megahertz (MHz) because of the undesirably limited frequency responses of the PNP differential input transistors.